Know Your Projection!

The Know Your Projection! tool provides a comprehensive coordinate diagnostics, automated Philippine Coordinate Reference System (CRS) candidate detection, and 2D affine georeferencing environment for vector datasets with missing, misidentified, or arbitrary local coordinate systems.

Access

• Menu: Gemma → Updating of Boundaries → Know Your Projection!
• Processing Toolbox: GMD Pipeline → 1Map → Know Your Projection!
• Algorithm ID: gmd_pipeline:projection_finder

When executed inside QGIS Desktop, the algorithm opens the interactive Know Your Projection! toolkit dialog. When run headlessly (e.g. via qgis_process), it performs a non-interactive coordinate diagnostic audit on the input vector layer and logs extent metrics and candidate CRS matches.

When to Use

Use this tool when:

• Vector shapefiles or GeoPackages have missing .prj definition files, invalid CRS tags, or unassigned projections.
• Boundary layers digitized in local CAD units (~0 to ~100,000) or legacy millimeter coordinates need to be georeferenced to standard Philippine CRS grids.
• Layer boundaries fail to overlay correctly onto satellite basemaps due to datum offsets between WGS 84, PRS92, and Luzon 1911.
• You need to audit coordinate bounding boxes and classify CRS regimes across municipal vector datasets prior to boundary harmonization.

Parameters

Inputs

Parameter	Type	Description
Input vector layer to diagnose (optional)	Feature Source (Any Geometry)	Optional vector layer to audit or diagnose. If omitted in Desktop mode, the interactive tool opens for workspace layer selection.

Outputs

Output	Type	Description
Diagnostic summary	String	Detailed coordinate classification, extent metrics, and candidate CRS matches logged during processing.

How It Works

1. Coordinate Extent Diagnosis:

   • Classifies bounding box magnitudes and spans into four distinct coordinate regimes:
     • Geographic Coordinates (Degrees): Coordinates within $[-{180}^{\circ },{180}^{\circ }]$ longitude and $[-{90}^{\circ },{90}^{\circ }]$ latitude. Suggests EPSG:4326 (WGS 84), EPSG:4683 (PRS92), or EPSG:4253 (Luzon 1911).
     • Projected Grid (Metres): Coordinates within $[100,000,1,000,000]$ Easting and $[300,000,2,700,000]$ Northing, matching Philippine Transverse Mercator (PTM) or Universal Transverse Mercator (UTM) zones.
     • Web Mercator (EPSG:3857): Coordinates within $[12,000,000,14,500,000]$ Easting.
     • Local / Arbitrary Grid: Non-georeferenced coordinates digitized from CAD drawings, millimeter tablet digitizers, or uncalibrated local surveys.

2. Automated Philippine CRS Candidate Scanning:

   • Tests layer extents across 23 national and regional coordinate definitions:
     • WGS 84: Geographic (EPSG:4326), UTM Zone 51N (EPSG:32651), UTM Zone 50N (EPSG:32650), UTM Zone 52N (EPSG:32652), Web Mercator (EPSG:3857).
     • PRS92: Geographic (EPSG:4683), PTM Zones 1–5 (EPSG:3121–3125), and UTM Zones 50N–52N (ESRI:102456–102458).
     • Luzon 1911: Geographic (EPSG:4253), PTM Zones I–V (EPSG:25391–25395), and UTM Zones 50N–52N (ESRI:102453–102455).
   • Identifies candidate systems that place the layer boundaries fully within the Philippine terrestrial extent (${116.0}^{\circ }\text{E}-{127.5}^{\circ }\text{E}$, ${4.0}^{\circ }\text{N}-{21.5}^{\circ }\text{N}$).

3. 2D 6-Parameter Affine Georeferencing:

   • When layers exist in arbitrary CAD coordinates, the tool calculates a 2D affine transformation matrix via Ordinary Least Squares (OLS) between digitized local coordinate pairs $(x,y)$ and verified reference control point coordinates $(X,Y)$:$$\begin{array}{rl}X& =a\cdot x+b\cdot y+c\\ Y& =d\cdot x+e\cdot y+f\end{array}$$
   • Solves for translation $(c,f)$, independent $X$ and $Y$ scale factors, rotation angles, and shear anisotropy to absorb non-uniform digitizing distortion.
   • Evaluates root-mean-square (RMS) residuals in metres across all control points to verify positional accuracy before applying changes.

4. Visual Basemap Inspection:

   • Automatically loads Google Satellite or Google Hybrid XYZ tile basemaps at the base of the layer tree.
   • Enables immediate visual verification of boundary alignment against real-world road networks, coastlines, and physical topography.

5. Direct Layer Reprojection & Export:

   • Allows users to reassign the confirmed CRS to the active layer or export georeferenced features into a standardized new GeoPackage in the target CRS (e.g. WGS 84 / UTM Zone 51N).

Supported Coordinate Systems Matrix

CRS Identifier	Name / Grid System	Typical Area of Use
EPSG:4326	WGS 84 (Geographic)	Global GPS default, web portals, interchange
EPSG:4683	PRS92 (Geographic)	National standard Philippine geographic datum
EPSG:4253	Luzon 1911 (Geographic)	Historical Philippine geographic datum
EPSG:32651	WGS 84 / UTM Zone 51N	Nationwide projected default in metres (Central Philippines)
EPSG:32650	WGS 84 / UTM Zone 50N	Western Philippines (Palawan, Kalayaan)
EPSG:32652	WGS 84 / UTM Zone 52N	Eastern Philippines (Eastern Mindanao, Siargao)
ESRI:102457	PRS92 / UTM Zone 51N	National PRS92 projected coverage on UTM grid
ESRI:102456	PRS92 / UTM Zone 50N	Western PRS92 UTM coverage
ESRI:102458	PRS92 / UTM Zone 52N	Eastern PRS92 UTM coverage
ESRI:102454	Luzon 1911 / UTM Zone 51N	Historical cadastral datasets on UTM grid
ESRI:102453	Luzon 1911 / UTM Zone 50N	Historical Palawan cadastral datasets
ESRI:102455	Luzon 1911 / UTM Zone 52N	Historical Eastern Mindanao cadastral datasets
EPSG:3121 – 3125	PRS92 / Philippines Zones 1 – 5	Official national PTM municipal survey grids
EPSG:25391 – 25395	Luzon 1911 / Philippines Zones I – V	Historical cadastral and NAMRIA topographic maps
EPSG:3857	WGS 84 / Pseudo-Mercator	Web tiling, OpenStreetMap, and Google basemaps

Supported Geometry Types

• Point and MultiPoint
• LineString and MultiLineString
• Polygon and MultiPolygon

Recommended Production Projection

For municipal boundary updating and spatial analysis, EPSG:32651 (WGS 84 / UTM Zone 51N) or the appropriate local PTM zone (EPSG:3121–3125) is recommended. Metric planar coordinates allow accurate area calculations, buffer generation, and geometric topology operations without ellipsoidal distortion.

Headless Diagnostics

When run headlessly without a GUI (such as via Python scripts or qgis_process run gmd_pipeline:projection_finder), the tool performs automated bounding box diagnosis and logs candidate systems directly to the processing console without opening the interactive dialog.